Circuit breakers incorporating a magnetic arc chute with an electromagnet



June 1964 G o. PERKINS ETAL 3,138,687

CIRCUIT BREAKERS INCORPORATING A MAGNETIC ARC CHUTE WITH AN ELECTROMAGNET Filed May 12, 1961 3 Sheets-Sheet l INVENTORS GORDON O.PERKINS GERALD J.EMER|CK ATTORNEY 3 Sheets-Sheet 2 GORDON O. PERKINS GERALD J EMERICK ATTORNEY June 23, 1964 G. o. PERKINS ETAL CIRCUIT BREAKERS IN CORPORATING A MAGNETIC ARC CHUTEI WITH AN ELECTROMAGNET Filed May 12, 1961 FIG. 2

June 1964 G. o. PERKINS ETAL 3,133,637

CIRCUIT BREAKERS INCORPORATING A MAGNETIC ARC CHUTE WITH AN ELECTROMAGNET Filed May 12, 1961 3 Sheets-Sheet 3 FIG. 3 g

INVENTORS GORDON O-PERKINS GERALD J. EMERICK 75, 5 ZQWQL.

ATTORNEY United States Patent 3 138 687 CIRCUET BREAKERS lNCORPORATING A MAGNETIC ARC CHUTE WITH AN ELECTRQMAGNET Gordon 0. Perkins, Cannonsburg, Pa., and Gerald J.-

The present invention relates to circuit breakers and particularly to circuit breakers of the type having socalled magnetic arc chutes.

The are drawn by a circuit breaker when opened under short-circuit conditions tends to form a loop which expands rapidly, because of self-induced electrodynamic effects. Arc chutes are placed in the path of the expanding arc, and the magnetic arc chute uses an electromagnet to augment the self-induced tendency of the loop to expand.

Two types of arc-current excited electromagnets have been used in arc chutes heretofore, which may be loosely designated the U-chute and H-chute, according to the cross-sectional shape of the magnetic structures involved. These two types of chutes have been designed simply to provide a loop-accelerating magnetic field.

An object of the present invention resides in the provision of a circuit breaker having a new and improved magnetic are chute.

The present invention is concerned with a departure from the elemental concept of loop-acceleration in an arc chute. The illustrative arc chute disclosed in detail below has three arc-excited electromagnets, including a U- shaped unit at each end of the arc chute and an H-shaped unit at the center. The three arc-excited magnets are so arranged that U-magnets are energized before the H- magnet is energized. The extremities of the arc are thus initially exposed to a maximum of arc-displacing field. At a higher level in the arc chute, the arc reaches and energizes the H-magnet and then the arc is exposed to the field of the H-shaped magnet that is centered between the U-shaped units. This manipulation of the are as it enters and is driven up through the splitter plate assembly has been demonstrated to be extremely effective compared to a similar arc chute having only an H-magnet in the splitter-plate assembly and having a rating of 500 kva. interrupting capacity. The illustrative arc chute construction with a mere one-third increase in the number of splitter plates used has attained twice that interrupting capacity. Neither an oversized H-magnet nor two oversized U-magnets proved successful. The extremities of the are are evidently subjected to the quenching effects of the arc-splitter plates very nearly in time with the exposure of the arc to the center portion of the splitterplate assembly. The performance of the novel arc chute is particularly effective as demonstrated by the complete elimination of so-called arc snufier structures at the top of arc chute for guarding against arc restrike occurring due to ionized gas emerging. The effectiveness of such novel are chute is also demonstrated by the extremely short arc-interruption times realized. The novel are chute is also notable for a minimized amount of destructive energy generated during the arcing time. Using the novel concepts here involved, less than one-cycle interrupting time is realized in a 15,000-volt circuit breaker having a 1,000,000 volt ampere interrupting capacity.

The use of the series of three arc-excited electromagnets in the disclosed arc chute attains certain further advantages. Each coil is operated at the instantaneous potential of that part of the arc to which it is connected, and the associated iron structures extend over only part of its length, horizontally. This construction simplifies the insulation requirements in each electromagnet.

The nature of the invention and its various further objects and features of novelty will be better appreciated from the following description in detail of an illustrative embodiment of the invention in its various aspects. This embodiment is shown in the accompanying drawings forming part of this disclosure. In the drawings:

FIG. 1 is a side view of an illustrative circuit breaker embodying features of the present invention;

FIG. 2 is an end view of the circuit breaker in FIG. 1 as viewed from the left-hand side of FIG. 1;

FIG. 3 is a top plan view of the arc chute of the circuit breaker in FIG. 1 drawn to larger scale;

FIG. 4 is a vertical cross section of the arc chute of the circuit breaker in FIG. 1, as viewed from the plane 44 in FIG. 3; and

FIG. 5 is a vertical cross section of the arc chute of FIG. 4 as viewed from the plane 5-5 in FIG. 4.

Referring now to the drawings, a circuit breaker is shown (FIG. 1) having a pair of stationary conductors or terminals 10 and 12 which are bridged by a moving contact arm 14. Pivot 16 forms an electrical and mechanical connection from moving contact arm 14 to conductor 12. Main moving contact 18 and moving arcing contact 20 make connection from the moving contact arm 14 to the companion main and arcing contacts of conductor 10.

By means of a mechanism not shown, moving contact arm 14 is held in the position shown when the circuit breaker is closed and, under overload conditions, the mechanism operates a shaft 22 of insulation to drive the moving contact assembly counterclockwise about pivot 16 as viewed in FIG. 1. Arcing contact 20 assumes the position represented in broken lines in FIG. 1 when the circuit breaker is open. In the process of this parting of the contacts, a spring mechanism 24 acts on the arcing contact 20 so as to cause that arcing contact to pivot clockwise relative to the remainder of the moving contact arm 14, arcing contact 20 moving through a limited angle about its pivot 26. This angle is limited by engagement of a nut 28 carried by the rod 30 that extends to a pivot 32 on arcing contact 20.

It is desirable to produce an are that is prominently looped upward when contact 20 parts from its companion arcing contact 34. For this reason the latter contact is formed to provide a vertical current path extending downward to its point of connection 36 to the structure united to terminal 10. A layer of insulation 38 provides solid backing between stationary arcing contact 34 and the remainder of the terminal structure.

Because of the above configuration, the current that traverses the arcing contacts follows an up-and-down loop which, at the instant of contact-parting, produces an are that curves upward prominently. Such an arc in the form of a loop tends to expand upward rapidly, and the arc thus has a strong tendency to rise into the arc chute above it.

The same current loop which causes the formation of the upward looped arc is also instrumental in producing a tendency of arcing contact 20 to lift mechanically from its companion contact 36 or blow off during shortcircuit intervals and before moving contact assembly 14 is released. This could be damaging. A compensating blow-on electrodynamic force is produced by a conductor 40 which is flexible and is in the form of a tight loop that carries current from the moving contact arm 14 to the arcing contact 20. Pivot 26 is insulated. A mechanical back-element 42 provides reaction for the expanding force in the current loop 40 and as a result of this arrangement, a powerful clockwise electrodynamic force applied to arcing contact 20 is developed about pivot 26,

which overcomes the above mentioned tendency of contact to blow away from its companion contact 34.

The are loop expands upward into the arc chute above the contacts. In FIG. 2 the arc chute is seen to include a pair of side plates 44 and 46 of insulation which are reinforced across their top zone by insulating plates 48. The chute is extended below the bottom edges of plates 44 and 46 by a further pair of insulating plates or skirts 50 that are spaced apart to provide operating space for contact arm 14. End plates 52 and 54 of insulation complement side plates 44 and 46 to form a sort of box with an open top and bottom. The inside surfaces of plates 44 and 46 are covered with a lining 56 (FIG. 2) of glassy insulation up to the bottom edges of the arc splitter plates 58. These lowermost edges are spaced an appreciable distance above skirts 50 (FIG. 4) the upper edge of which is just above the arcuate path of movable arcing contact 20.

The left-hand portion of the arc chute in FIGS. 1 and 4 includes a coil 60 having a lead 62 that extends to a metal bar 64 providing a connection from coil 60 to terminal 12. The opposite end of coil 60 is joined to an arc runner 66. Coil 60 is contained within a space protected from the arc and hot arc gases by walls 68, while arc runner 66 is freely exposed to the internal space 67 of the arc chute below the splitter plates.

Coil 60 has a core 70 of laminated iron extending perpendicular to and through side walls 44 and 46 of the arc chute and they abut identical laminated iron polepieces 74 (FIG. 2) and at the opposite faces of the arc chute. These pole pieces each includes a lower portion 74a which flanks arc runner 66 (FIG. 4) and an upper portion 74b which flanks the upward extension 66b of the arc runner and about 25% of the left-hand group of arc splitter plates 58 (FIG. 4).

Coil 60 is formed of a number of turns, usually not more than 5. Non-magnetic plates 76 as of stainless steel that are held together by clamping bolts (not shown) restrain coil 60 against violent expansion during shortc1rcu1t current surges.

At the right-hand side of the arc chute is another coil 78 having one terminal 80 connected to one terminal 10 of the circuit breaker and the opposite terminal of coil 78 is connected to an arc runner 82. Core 84 within coil 78 has its ends in abutment with lower portions 86a of a pair of pole pieces 86 disposed against the outside faces of insulating walls 44 and 46, pole pieces 86 having upper portions 86b which flank the upward extension 82b of arc runner 82, plus about 25% of the right-hand group of are splitter plates 58. If cross-sections through cores 70 and 84 perpendicular to arc runners 66 and 82 were taken, those cores plus the extending poles would be roughly U-shaped, and as a term of reference coil 60 and magnetic structure 70, 74 may be called a U-magnet, a term also appliable to coil 78 and magnetic structure 84, 86. I

The arc splitter plates 58 are separated into two groups by a structure which may be called an H-magnet. This structure includes a coil 90 that is insulated and mechanically reinforced by clamps, coil 90 having end connections 92 and 94 to the lower ends or horns 96 and 98 of arc runners 100 and 102. An arc extinguishing device 104 is disposed between arcing horns 96 and 98. Device 104 consists of an assembly of Vertical insulating plates joined to an overlying horizontal insulating plate as shown in FIG. 4. A magnetic structure is associated with coil 90, including a core 106 which projects through the side plates 44 and 46 of the arc chute and pole pieces 108 extending to the right and left of both opposite end portions of core 106. The horizontal cross section of the magnetic structure 106, 108 resembles the letter H. The pole pieces 108 confront about half of the arc splitter plates in each of the two groups shown to the right and the left of the coil-and-core assembly 90, 106.

Pole pieces 108 are held together, as units, and the units are held against the core 106, by bolts extending through top and bottom brackets 110, which brackets also secure the pole pieces to the arc chute itself.

The are splitter plates 58 as seen in FIG. 5 involve lower edges 58a which have successive aligned sloping edges 58b that are symmetrical about the center line of the arc chute in FIG. 5. Each plate has a laterally sloping slot 580 which extends to an upper limit 58d. Slots 58c in successive splitter plates 58 slant in opposite directions, alternately from each plate to the next along the arc chute. The upper ends of the slots 580 are roughly in alignment with the upper ends of the pairs of poles 74 and 86 of the U-magnets and with the top of the magnet structure 106, 108 of the H-magnet.

Plates 44 and 46 of insulation form side walls of the arc chute. Arcing plates 58 are separated from each other by spacers and/or spacing cement between the vertical edges of plates 58, such separating structure being of insulating material and serving to augment the side-wall insulation provided. This insulation separates the arc-containing spaces between arc-splitter plates 58 and the pole pieces 74, 86 and 108 which flank sidewalls 44 and 46. The magnetic-pole structures, in turn, are insulated from their respective arc-current excited coils by heavy fish paper or the like.

Arcing horn 66a is the part of arc runner 66 that is nearest to the arcuate path of arcing tip 20 and are runner 66 has an upward extension 66b along the end-wall of the arc chute. Arc runner 82 has a similar arcing horn 82a and vertical extension 8% against the opposite endwall of the are chute. Arc runner extensions 66b and 82b of the U-magnets and arc runners 108 and 182 associated with the H-magnet extend to a level slightly higher than slot-ends 580. The upper ends of the spaces between plates 58 are open and unobstructed.

In operation, the moving contact assembly 14 swings counterclockwise when the circuit breaker is opened, and in the event that the opening ocurs in response to automatic overcurrent or fault-current release of the circuit breaker, an arc is struck between arcing contacts 20 and 34. This are is in the form of an upwardly directed loop which tends to expand rapidly and rise as it expands. The extremities of the loop reach arcing horns 66a and 82a. The are transfers to arc runners 66 and 82, and the arcing current then flows in a path from terminal 10 of the circuit breaker through coil 78 to arc runner 82, as an are through the space below splitter plates 58 to are runner 66, through coil 68 to terminal 12. A magnetomotive force is developed in cores 70 and 84 which produces a strong arc-accelerating magnetic field in the immediate region of the arc runners 66 and 82. The natural tendency of the ends of the arc to speed along the surfaces of the arc runners is thus considerably augmented by the two U-magnets 60, 74, 74a and 78, 84 and 86a.

As the loop continues to rise and expand, the central portion of the loop is intercepted by arcing horn 98 and then by arcing horn 96. The portion of the are between these arcing horns is extinguished by are splitter plates 104. In this way, coil 90 of the H-magnet is injected in series with the arc, which is thus divided into two long segments extending from arc runner 66 to arcing horn 96 and from arcing horn 98 to are runner 82. The magnetomotive force produced by the arcing current traversing coil 90 produces a magnetic field parallel to the arc splitter plates between pole pieces 108 at opposite faces of the arc chute.

The magnetic structure associated with coil 60 is connected to connector 62 at the left-hand end of the circuit breaker, so that magnetic structure 78, 74 is at the same instantaneous potential as circuit breaker terminal 12. The right-hand magnet has its structure connected to terminal of coil 78 so as to operate at the same instantaneous potential as circuit breaker terminal 10. Coil in series with the arc segments is substantially mid-way between the extreme instantaneous potentials of terminals and 12 and its magnet structure may be considered as operating at the mid-potential point of the arc.

Inasmuch as each magnetic structure extends across only a small part of the total length of the arc chute, the insulating wall of the arcing chute is subjected to only a corresponding small part of the total potential difference that may be developed across the whole circuit breaker. Typically, circuit breakers of this construction are designed for alternating current at 13,800 volts under short-circuit conditions and somewhat higher voltage under conditions where the arcing current is less than the maximum interrupting capacity of the breaker.

Because of this construction, the insulation requirements between each magnet structure and the arcing space within the arc chute is considerably less than would be the case if the arc chute were equipped with only an H-magnet or a U-magnet.

The advantages of an H-magnet are realized in that the arc is accelerated upward over a broad region, approximately half the horizontal dimension of the arc chute, without requiring the lateral edges of the H-magnet pole pieces to be so far from its core as to provide inadequate arc-accelerating field strength. The resulting field strength is not so great at any particular part of the arc chute as to drive gas out through the top of the chute while still ionized. If that were to happen, restrike could take place with resulting failure of the circuit breaker to interrupt the circuit. The disclosed arc chute is highly efficient in this respect, as is evident from the fact that so called snutfers that heretofore have been used to insure quenching of residual ionized gas at the top of known are chutes are not needed in a 1,000 kva. embodiment of the arc chute disclosed. This embodiment has been so successful that it holds promise for a much higher interrupting capacity. With an increased test range, this are chute could probably give the circuit breaker an interrupting capacity of 1500 kva.

Each coil 60, 78 and 90 has but five turns of copper strip in a practical embodiment, and each such coil individually involves only a moderate voltage drop that does not cause trouble in connection with transfer of arcing current into the respective coils. An excessive voltage drop might be occasioned across a coil if one were to attempt arc extinction through oversized arc-currentexcited U-magnets or an oversized H-magnet with many turns in the coil, and it would then be diflicult and it would involve complications to insure prompt transfer of the arcing current into the coil.

The coils of all the magnets in the chute shown are related to the direction of the arc current so that the magnetic poles at side sheet 44 are instantaneously all alike in polarity, as is also true of the poles at side sheet 46. The fringing fields of the pole pieces 74b and 108 add to each other in the region of splitter plates 58 between the edges of such poles, and similarly the fringing fields between pole pieces 86b and 108 add up in the arcing spaces of splitter plates 58 between the edges of those pole pieces 86b and 108. Accordingly, the arc-accelerating magnet field is uniformly effective, to a high degree, because of the sequence of arc-excited electromagnets each of which is of modest proportions and which are separated from each other to a limited extent so that all parts of the arc chute are subjected to adequate arcaccelerating magnet fields.

The nature of the invention and the various features involved in the foregoing embodiment are naturally susceptible to a latitude of modification, rearrangement, and varied application by those skilled in the art. Consequently the invention should be broadly construed in accordance with its full spirit and scope.

What I claim is:

1. A circuit breaker having a pair of terminals, and including an arcing contact movable in an arcuate path during opening of the circuit breaker and a companion arcing contact engageable by said movable arcing contact when the circuit breaker is closed, and an arc chute disposed adjacent to and above said arcuate path, said arc chute including an assembly of vertical arc splitter plates whose lower edges are spaced above and disposed transverse to said arcuate path, three electromagnets including respective coils arranged to be excited by arcing current, a pair of downwardly converging arc runners extending from the ends of said assembly of arc splitter plates into close proximity to said arcuate path, the coils of two of said electromagnets each having a first terminal connected to a respective one of said arcing contacts and said coils of said two electromagnets each having a second terminal connected to a respective one of said are runners, said two of said electromagnets having pole pieces flanking said arc runners and the lateral end portions of said assembly of arc splitter plates and said pole pieces of each of said two electromagnets being separated from the pole pieces of the other of said two electromagnets by a substantial distance to avoid directly imposing an accelerating magnetic field on the central part of an arc, and the third electromagnet having a coil and arcing horns connected to said coil and said third electromagnet having pole pieces flanking the central half of said assembly of arc splitter plates, said are runners being disposed substantially closer to said arcuate path than the arcing horns of said third electromagnet.

2. A circuit breaker having a pair of terminals, a pair of companion contacts and a moving contact arm carrying one of said contacts for interconnecting said terminals, said one contact being movable in an arcuate path into and out of engagement with the companion contact, and an arc chute disposed adjacent to and above said arcuate path, said arc chute including an assembly of vertical mutually spaced arc splitter plates whose lower edges are horizontally aligned and are spaced above and disposed transverse to said arcuate path and said arc chute having confronting vertical side walls extending across said are splitter plates, said are chute including a pair of arc runners extending from respective points close to the ends of said arcuate path and diverging upwardly to the level of the lower edges of the arc splitter plates at the ends of said arc chute, a pair of coils each connected between a respective one of said are runners and a respective one of said terminals, a magnetic core through each of said coils and having pole pieces including portions flanking respective lateral end portions of the sidewalls of the arc chute and including portions flanking said are runners below the level of the arc splitter plates and extending relatively close to the ends of said arcuate path and the pole pieces of each of said pair of electromagnets being spaced substantially from the pole pieces of the other of said pair of electromagnets by a substantial distance to avoid directly imposing an accelerating magnetic field on the central part of an arc, and a third coil centrally disposed in said arc chute and having arcing horns disposed only slightly below the lowermost edges of said are splitter plates but above the lowermost portions of said are runners and thus being more remote from said arcuate path, said third coil having a magnetic core and having magnetic pole pieces centered on the opposite side-walls of said are chute and extending across approximately half the width of the side-walls, each of the last-mentioned pole pieces being disposed between portions of the first-mentioned pole pieces and above other portions of the first-mentioned pole pieces.

3. An arc chute for a circuit breaker having a pair of companion contacts and a contact arm for interconnecting said terminals wherein one of said contacts is movable in an arcuate path, said are chute including an assembly of vertical arc splitter plates whose lower edges are spaced above and disposed transverse to said arcuate path and said arc chute having confronting vertical side walls extending across said are splitter plates, said arc chute including a pair of downwardly converging arc runners extending from lowermost edges of the outermost arc splitter plates and said are runners being mutual.- ly separated for disposition adjacent the extremities of the arcuate path of the movable circuit breaker contact, a pair of coils each connected to a respective one of said arc runners and having an end point for connection to a respective one of said terminals, a magnetic core through each of said coils and having pole pieces including portions flanking the respective lateral end portions of the side-walls of the arc chute and including portions flanking said arc runners below the level of the are splitter plates,-and a third coil centrally disposed insaid are chute and having arcing horns disposed only slightly below the lowermost edges of said arc splitter plates but above the lowermost portions of said arc runners so as to be more remote from the arcuate path of the movable circuit breaker contact, said third coil having a magnetic core and magnetic pole pieces centered on the opposite side-walls of said are chute and extending across approximately half of the width of the side-walls, each of the last-mentioned pole pieces being disposed between portions or" the first-mentioned pole pieces and above other portions of the first-mentioned pole pieces.

4. An arc chute for a circuit breaker having an arcing contact movable in an arcuate path, said arc chute including an assembly of vertical arc splitter plates whose lower edges are spaced above and disposed transverse to said arcuate path, three electromagnets including respective coils arranged for excitation by arcing current, a pair of downwardly converging arc runners extending from the ends of said assembly of arc splitter plates into close proximity to said arcuate path, two of said electromagnets each having a coil connected to one of said arc runners, respectively, and each having pole pieces flankingsaid are runners and respective lateral end portions of said assembly of arc splitter plates, and the third of said electromagnets having pole pieces flanking the central half of said assembly of arc splitter plates, the third of said electromagnets having a coil and arcing horns extending from said coil and disposed substantially farther from said arcuate path than the lowermost ends of said are runners.

5. An arc chute for a circuit breaker having an arcing contact movable in an arcuate path and cooperable with a companion arcing contact, said are chute including an assembly of vertical arc splitter plates whose-lower edges are adapted to be disposed above and transverse to the path of said arcing contact, three electromagnets including respective cores and coils thereon, said coils having terminals disposed for excitation by arcing current, a pair of downwardly converging arc runners extending from the ends of said assembly of arc splitter plates below the lowermost edges of said splitter plates, said are runners each constituting one terminal of a respec tive coil of two of said electromagnets, said two electromagnets each having pole pieces flanking respective laterally outward parts of the arc splitter plate assembly and flanking said arc runners, and the third electromagnet having pole pieces disposed between the pole pieces of said two electromagnets and flanking the central portion of said assembly of arcsplitter plates.

6. An arc chute for a circuit breaker having a movable arcing contact and a companion arcing contact, said arc, chute including a two-section assembly of vertical arc splitter plates whose lower edges are adapted to be spaced above and disposed transverse to the path of the arcing contact, three electromagnets including respective cores and respective coils thereon, said coils having terminals disposed for excitation by arcing current, a pair of downwardly converging arc runners extending from the opposite ends of said assembly of arc splitter plates below the lowermost edges of said splitter plates, said are runners each constituting one terminal of a respective coil of two of said electromagnets, said two electromagnets each having pole pieces flanking a respective one of said are runners and the laterally outward one-quarter portions of respective sections of said two-section arc-splitter plate assembly, and the third electromagnet having .the coilthereof interposed between the two sections of arc splitter plates and having pole pieces flanking half of each section of said are splitter plate assembly between said laterally outward one-quarter portions.

7. An arc chute for a circuit breaker having a movable arcing contact and a companion arcing contact, said am chute including an assembly of vertical arc splitter plates, three electromagnets including a first electromagnet having an H-shaped magnetic structure flanking a large part of said arc-splitter plate assembly and including a coil having a pair of arcing horns, a pair of downwardly converging arc runners extending from the ends of said arc-splitter plate assembly to respective points close to the path of said arcing contact, the other two of said three electromagnets each having a U-shaped magnetic structure flanking a respective one of said are runners and said U-shaped magnetic structures being spaced from each other substantially so as to avoid directly imposing a substantial accelerating magnetic field on the central part of an arc, and each of said other two electromagnets having a coil connected to a corresponding one of said are runners.

References Cited in the fileof this patent UNITED STATES PATENTS 1,398,982 Tritle Dec. 6, 1921 1,713,229 Hewlett May 14, 1929 2,329,003 Seaman Sept. 7, 1943 2,378,124 Bolsterli June 12, 1945 2,635,158 Peters Apr. 14, 1953 2,664,479 Pokorny Dec. 29, 1953 2,816,992 Frink Dec. 17, 1957 2,901,579 Simpson Aug. 25, 1959 

1. A CIRCUIT BREAKER HAVING A PAIR OF TERMINALS, AND INCLUDING AN ARCING CONTACT MOVABLE IN AN ARCUATE PATH DURING OPENING OF THE CIRCUIT BREAKER AND A COMPANION ARCING CONTACT ENGAGEABLE BY SAID MOVABLE ARCING CONTACT WHEN THE CIRCUIT BREAKER IS CLOSED, AND AN ARC CHUTE DISPOSED ADJACENT TO AND ABOVE SAID ARCUATE PATH, SAID ARC CHUTE INCLUDING AN ASSEMBLY OF VERTICAL ARC SPLITTER PLATES WHOSE LOWER EDGES ARE SPACED ABOVE AND DISPOSED TRANSVERSE TO SAID ARCUATE PATH, THREE ELECTROMAGNETS INCLUDING RESPECTIVE COILS ARRANGED TO BE EXCITED BY ARCING CURRENT, A PAIR OF DOWNWARDLY CONVERGING ARC RUNNERS EXTENDING FROM THE ENDS OF SAID ASSEMBLY OF ARC SPLITTER PLATES INTO CLOSE PROXIMITY TO SAID ARCUATE PATH, THE COILS OF TWO OF SAID ELECTROMAGNETS EACH HAVING A FIRST TERMINAL CONNECTED TO A RESPECTIVE ONE OF SAID ARCING CONTACTS AND SAID COILS OF SAID TWO ELECTROMAGNETS EACH HAVING A SECOND TERMINAL CONNECTED TO A RESPECTIVE ONE OF SAID ARC RUNNERS, SAID TWO OF SAID ELECTROMAGNETS HAVING POLE PIECES FLANKING SAID ARC RUNNERS AND THE LATERAL END PORTIONS OF SAID ASSEMBLY OF ARC SPLITTER PLATES AND SAID POLE PIECES OF EACH OF SAID TWO ELECTROMAGNETS BEING SEPARATED FROM THE POLE PIECES OF THE OTHER OF SAID TWO ELECTROMAGNETS BY A SUBSTANTIAL DISTANCE TO AVOID DIRECTLY IMPOSING AN ACCELERATING MAGNETIC FIELD ON THE CENTRAL PART OF AN ARC, AND THE THIRD ELECTROMAGNET HAVING A COIL AND ARCING HORNS CONNECTED TO SAID COIL AND SAID THIRD ELECTROMAGNET HAVING POLE PIECES FLANKING THE CENTRAL HALF OF SAID ASSEMBLY OF ARC SPLITTER PLATES, SAID ARC RUNNERS BEING DISPOSED SUBSTANTIALLY CLOSER TO SAID ARCUATE PATH THAN THE ARCING HORNS OF SAID THIRD ELECTROMAGNET. 